Firstly could you please inform what temperature is indicated by "blue" on the thermal image.
Secondly it would be interesting to get the cyclone gas temperature (at outlet or inlet) for each stage.
Material clogging or build ups in the cone area could be caused by several conditions. The most likely one is sticking of volatiles (in combination with or without to shallow cone angle)
Volatiles can be originating from preheater gas from the lower stage cooling down. It can be expected that by high chloride/sulafate internal circulation and insufficient bypass (or no bypass) volatiles will clogg onto walls (especially with limited gas velocities).
For chloride those build ups can be expected in the temperature range between 650°C to 800°C and for Sulfate from 800°C to 1100°C. This is to be taken as a rough guide only it depends on the actual chemical composition of the volatiles.
Another reason may be open (not properly closing) pendelum flaps whereby hot fine meal is being transported through the meal pipe and diptube to the next stage upwards where it will cool down and cause clogging.
Of course there is still the possibility of wrong calibrated or malfunction of instruments.
